X-Git-Url: https://git.wirehaze.ovh/lifeboot.git/blobdiff_plain/9ed39df5a90be6cce8ee7948cefbf8e5fc7fe089..0d9586ff094ca3530341657b0ded38e747286de8:/lifeboot.asm?ds=sidebyside

diff --git a/lifeboot.asm b/lifeboot.asm
index 8b530f9..2d1542f 100644
--- a/lifeboot.asm
+++ b/lifeboot.asm
@@ -1,94 +1,122 @@
 bits 16
 org 0x7c00
 
-%define ORG 0x7c00
-%define VGA 0xb8000
+; =============================================================================
 
-%define COLS 80
-%define ROWS 25
+; settings --------------------------------------------------------------------
 
 %define DEAD ' '            ; char to represent a dead cell
 %define ALIVE '#'           ; char to represent an alive cell
 %define PRINT_COLOR 0x07    ; grey on black
+%define WAIT_DELAY 0x02     ; 0.131072 seconds
 
-%define GRID        ORG + 512               ; current cell grid (80 * 25 bytes)
-%define NEXT_GRID   GRID + COLS * ROWS      ; next cell grid (80 * 25 bytes)
-%define XSS         NEXT_GRID + COLS * ROWS ; xs() state
+; constants -------------------------------------------------------------------
+
+%define COLS 80
+%define ROWS 25
+%define VGAPGSZ (COLS * ROWS * 2)
+
+; memory layout ---------------------------------------------------------------
+
+%define ORG 0x7c00
+%define VGA 0xb8000
+%define DAT ORG + 512
+
+; static variables ------------------------------------------------------------
+
+%define xss DAT             ; xs() state (word)
+%define currvgapg xss + 2   ; current vga page near pointer (word)
+
+; =============================================================================
 
 ; entry point -----------------------------------------------------------------
 
 ; initialize segment registers
-xor ax, ax
+mov ax, VGA >> 4
 mov ds, ax
 mov es, ax
+
+xor ax, ax
 mov ss, ax
 
 ; set stack pointers
-mov sp, ORG
 mov bp, ORG
+mov sp, bp
 
 ; clear direction flag
 cld
 
-; clear screen ----------------------------------------------------------------
-
 ; disable cursor
-mov ch, 0x3f
-mov ah, 0x01
-int 0x10
+mov ch, 0x3f                ; cursor start and options
+mov ah, 0x01                ; set text-mode cursor shape
+int 0x10                    ; video services
 
-; clear video memory
-mov cx, COLS * ROWS
-mov ax, VGA >> 4
-mov es, ax
-xor di, di
-mov ax, (PRINT_COLOR << 8) | DEAD
+; initialize xss
+mov ah, 0x00                ; get
+int 0x1a                    ;  system time
+mov [xss], dx               ; cx:dx = number of clock ticks since midnight
 
-rep stosw                   ; fill cx words at es:di with ax
+; initialize currvgapg
+mov word [currvgapg], VGAPGSZ
 
 ; setup -----------------------------------------------------------------------
 
 setup:
 
-; initialize xss
-mov ah, 0x00                ; get
-int 0x1a                    ;  system time
-mov [XSS], dx               ; cx:dx = number of clock ticks since midnight
+call init_grid
+call flip_vga_page
+call delay
 
-; initialize grid
-mov cx, COLS * ROWS         ; for each cell
-xor ax, ax
-mov es, ax
-mov di, GRID                ; es:di -> GRID
+jmp setup
 
-.initgrid:                  ; do {
-    call xs                 ;     [XSS] = rand
-    mov al, DEAD            ;     al = DEAD (likely)
+; =============================================================================
 
-    test [XSS], 0b11
+; functions -------------------------------------------------------------------
 
-    jnz  .initgrid_nz       ;     if ([XSS] % 4 == 0)
-    mov al, ALIVE           ;         al = ALIVE
+; delay() - suspend program execution temporarily -----------------------------
 
-.initgrid_nz:
-    stosb                   ;     [es:(di++)] = al
+; clobbers ah, cx, dx
 
-    loop .initgrid          ; } while (--cx)
+delay:
 
-jmp halt
+mov cx, WAIT_DELAY          ; cx:dx = interval in microseconds
+mov dx, 0
 
-; functions -------------------------------------------------------------------
+mov ah, 0x86
+int 0x15                    ; wait
+
+ret
+
+; vsync() - wait for display to enter the next VBlank cycle -------------------
+
+; clobbers ax, dx
+
+vsync:
+
+mov dx, 0x3da               ; input status #1 register
+
+.wait_on:
+    in al, dx
+    test al, 0x08           ; vertical retrace bit
+    jnz .wait_on
+
+.wait_off:
+    in al, dx
+    test al, 0x08
+    jz .wait_off
+
+ret
 
 ; xs() - xorshift pseudorandom number generator -------------------------------
 
 ; output:
-; [XSS]     = updated state
+; [xss]     = updated state
 
 ; clobbers bx, dx
 
 xs:
 
-mov bx, [XSS]
+mov bx, [xss]
 mov dx, bx
 
 shl dx, 1                   ; dx = xss << 1
@@ -101,43 +129,61 @@ mov dx, bx
 
 shl dx, 10                  ; dx = xss'' << 10
 xor bx, dx                  ; bx = xss'' ^ (xss'' << 10)
-mov [XSS], bx
+mov [xss], bx
+
+ret
+
+; init_grid() - initialize grid cells randomly --------------------------------
+
+; clobbers ax, bx, cx, dx, di
+
+init_grid:
+
+mov ah, PRINT_COLOR         ; ah = color attribute
+mov di, [currvgapg]         ; es:di -> grid start
+mov cx, COLS * ROWS         ; for each cell
+
+.write_cell:                ; do {
+    call xs                 ;     [xss] = rand
+    mov al, DEAD            ;     al = DEAD (likely)
+
+    test word [xss], 0b11
+
+    jnz  .nz                ;     if ([xss] % 4 == 0)
+    mov al, ALIVE           ;         al = ALIVE
+
+.nz:
+    stosw                   ;     [es:di] = ax ; di += 2
+
+    loop .write_cell        ; } while (--cx)
 
 ret
 
-; errors ----------------------------------------------------------------------
+; flip_vga_page() - flip active display page ----------------------------------
+
+; clobbers ax, dx
+
+flip_vga_page:
 
-hello:
-mov si, str.hello
+call vsync
 
-; print the error message string in si and halt
-; note: we assume es = VGA_SEG and ds = 0
-printerr:
-xor di, di
-mov ah, PRINT_COLOR
+xor word [currvgapg], VGAPGSZ   ; flip currvgapg
+setnz al                        ; al = !(currvgapg == 0)
 
-; es:di = video memory
-; ds:si = error message
-; al = current char
-; ah = color attribute
+mov ah, 0x05                ; select active display page
+int 0x10                    ; video services
 
-write_char:
-lodsb                   ; al = [ds:si], si += 1
-or  al, al              ; on null terminator,
-jz  halt                ;  halt
-stosw                   ; [es:di] = ax, di += 2
-jmp write_char
+ret
+
+; halt() - stop program execution ---------------------------------------------
 
 halt:
-cli                     ; disable interrupts
+
+cli                         ; disable interrupts
 hlt
 jmp halt
 
-; data ------------------------------------------------------------------------
-
-str:
-.hello:
-    db "lifeboot", 0
+; =============================================================================
 
 times 510 - ($ - $$) db 0   ; fill remaining bytes with zeroes
 dw 0xaa55                   ; mbr magic byte